A coherence length discriminator (CLD) is a device that produces intensity variations (fringes) for radiation having a coherence length substantially longer than that of the CLD, referred to as coherent radiation, while producing little or no intensity variations for radiation having a coherence length substantially shorter than that of the CLD, referred to as incoherent radiation. Any unequal path interferometer may function as a CLD. Examples include a Fabry-Perot etalon, a Fizeau interferometer, Michaelson (parallel or filter mirrors) interferometers, and other interferometer types. The optical path difference (OPD) between reflectors determines the coherence length.
The detection of pulsed laser radiation in the presence of background radiation is described in U.S. Pat. No. 4,536,089, "Analyzer for Coherent Radiation", issued Aug. 20, 1985 to the present inventor, and also in U.S. Pat. No. 4,309,108, "Analyzer for Coherent Radiation", issued Jan. 5, 1982, also to the present inventor. In these U.S. Patents, a multi-stepped Fabry-Perot etalon is positioned in front of detector channels. For coherent illumination, the transmission of the etalon is a function of OPD. Step heights are chosen to have an OPD of approximately one half wavelength.
Reference is also made to U.S. Pat. No. 4,170,416, "Apparatus for Analyzing Coherent Radiation", issued Oct. 9, 1979 to Carroll R. Fencil, wherein a Fabry-Perot etalon has regions of different thickness and is disposed between a source of coherent radiation and detector elements. Modulation with respect to the source of coherent radiation is effected by angular movement of the entire unit, by mounting the unit in a moving vehicle, aircraft, or rotating mechanisms.
Reference is also made to the teaching of U.S. Pat. No. 4,806,747, issued Aug. 15, 1989, entitled "Optical Direction Sensor Having Gray Code Mask Spaced from a Plurality of Interdigitated Detectors", and U.S. Pat. No. 4,806,747, issued Feb. 21, 1989, entitled "Optical Direction of Arrival Sensor with Cylindrical Lens", both to Dunavan et al. These patents teach optical direction sensors.
U.S. Pat. No. 3,824,018, "Coherent Light Source Detector", issued Jul. 16, 1974 to Robert Crane, Jr. describes a Fabry-Perot etalon that is mounted so as to be pivotable or rotatable about an axis parallel to its surfaces. The entire etalon rotates about the axis Radiation transmitted by the etalon falls on a radiation sensitive surface of a radiation detector. The detector may rotate with the etalon or may be stationary. In one embodiment a surface of the etalon has two regions, each region being spaced by a different amount from an opposite surface. The spacing is said to be an exact odd multiple of quarter wavelengths of the coherent radiation being detected, divided by the index of refraction.
Although well suited for their intended uses, the teaching of these patents are not readily compatible with pulse detection techniques. Also, detector apparatus employing the teaching of these patents are not immune to atmospheric scintillation, detector nonuniformities, and detector microphonics. Furthermore, these and other techniques may require that a target or platform be in motion so as to generate modulation, or that the laser detector etalon be rotated or scanned through relatively large angles in order to modulate the CW signal. Typically, the resultant modulation is restricted to low frequencies and also requires bearings or high angle flexures, having a limited life, in order to achieve the modulation. In addition the typically large excursions require high power.
Furthermore, previous CW detector approaches have not exhibited a compatibility with pyroelectric detectors. This type of detector is advantageous in that it does not require cooling. However, pyroelectric detectors suffer from microphonics, especially at the low modulation frequencies obtained from conventional CW detector assemblies.
It is also noted that while the previous teachings may show a plurality of etalon elements and detector elements, a given detector element A is always associated with an etalon element A. That is, these previous teachings do not show an etalon translated in such a manner as to move an etalon element B relative to a detector element A.
It is thus one object of the invention to provide method and apparatus that overcomes these deficiencies while improving an ability to detect CW coherent radiation.
It is another object of the invention to provide a CW detector assembly having a higher modulation frequency than that obtained previously and that subtracts out microphonic noise, while adding desired signals, thus providing compatibility with pyroelectric detectors.
Another object of the invention is to provide a CW radiation detector assembly having significantly reduced mechanical motions, relative to conventional CW detectors, and that furthermore provides a high degree of mechanical isolation.
It is a further object of the invention to provide CW wavelength and Direction of Arrival (DOA) capability in conjunction with a quad cell, bicell or gray code techniques.
It is another object of the invention to provide apparatus for detecting CW laser radiation, the apparatus exhibiting a wide Field of View (FOV), a wide spectral passband, high coherence discrimination, a compatibility with existing pulse detection technology, an immunity to atmospheric scintillation, and an insensitivity to detector nonuniformities and microphonics.
It is one still further object of the invention to provide apparatus having the above mentioned features for detecting a CW laser signal that is buried below an incoherent background through the use of background subtraction techniques.